We believe in Scripture and Tradition, both of which are divinely inspired. As the Scriptures themselves say 2 Thess 2:15 Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours.
Both are handed on in all their integrity in the Church, by virtue of Jesus’ promises to send the Holy Spirit to lead us in all truth. The letter from the Pope linked above is an example of the successor of Peter confirming this truth, not inventing it. The letter itself lays out some of the evidence for this belief from the beginning.
The early saints bear witness to Tradition and its consistency over time and are also generally good sources of Christian wisdom. But they provide no new revelation. They are not inspired like the writers of Scripture or the Apostles.